Feedback circuit for anodizing thin-film resistors

ABSTRACT

A THIN-FILM RESISTOR, WHICH IS TO BE ANODIZED TO INCREASE ITS RESISTANCE TO A PRESELECTED VALUE IS CONNECTED TO ONE ARM OF A BRIDGE CIRCUIT. AS THE RESISTOR IS BEING ANODIZED AND INBALANCE INTHE BRIDGE DRIVES A MONITOR CIRCUIT WHICH CONTROLS A VARIABLE RESISTANCE IN SERIES WITH A SUPPLY OF ANODIZING VOLTAGE. AS THE THIN-FILM RESISTOR BEING ANODIZED APPROACHES THE PRESELECTED VALUE, THE BRIDGE BECOMES MORE NEARLY BALANCED AND THE VARIABLE RESISTOR INCREASES IN VALUE TO INTERRUPT THE FLOW OF ANODIZING CURRENT.

May 4, 1 971 F. H. BLITCHINGTON, JR 3,577,335

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United States Patent Office Patented May 4, 1971 Filed May 9, 1968, Ser. No. 727,948 Int. Cl. B01k 3/ 00 U.S. Cl. 204--228 6 Claims ABSTRACT OF THE DISCLOSURE A thin-film resistor, which is to be anodized to increase its resistance to a preselected value is connected to one arm of a bridge circuit. As the resistor is being anodized, an imbalance in the bridge drives a monitor circuit which controls a variable resistance in series with a supply of anodizing voltage. As the thin-film resistor being anodized approaches the preselected value, the bridge becomes more nearly balanced and the variable resistor increases in value to interrupt the fiow of anodizing current.

CROSS-REFERENCE TO RELATED APPLICATION This application is related to an application filed in the name of A. R. Gerhard on May 9, 1968, Ser. No. 727,972.

BACKGROUND OF THE INVENTION (1) Field of the invention The invention relates to a control circuit for anodizing thin-film resistors. One problem encountered in anodizing resistors is that precise resistor values cannot be attained if the resistance is not monitored during the anodization process. Further, in anodizing resistors on a production basis, it is desirable to apply a high anodizing current at first and then reduce the current as the value of the resistor approaches the desired resistance, in order to decrease the total time required for anodization.

(2) Description of the prior art In the past, a number of different circuits have been developed in which the resistance of a resistor being anodized is monitored by a bridge circuit to stop the flow of anodizing current when the bridge is balanced. Such anodization control circuit may, additionally, decrease the anodizing current as the resistor approaches a desired value. However, in these prior circuits the resistor is alternately connected to the monitoring and anodizing circuits, so that part of the time the resistance is being rneasured and the remaining time the resistor is being anodized. Since the resistor is only anodized during part of the time, the total process of anodization takes longer. Further, the resistance of the resistor being anodized may pass through the desired value during the last cycle of anodization and be outside the desired tolerance range. Finally, if the resistance of the resistor being anodized is continuously monitored during anodization, such as by a Wheatstone bridge, the anodizing current flowing in the resistor disturbs the operation of the bridge and results in errors.

SUMMARY OF THE INVENTION The object of the invention is a new and improved anodizing control circuit. In one embodiment of the circuit, a resistor to be anodized is continuously connected to a monitor circuit that produces a control voltage signal which is in proportion to the deviation of the resistor from a desired value. A source of anodizing current is connected to the resistor via an electrolyte to continuously anodize it and increase its resistance. A control circuit, which is connected to the source of anodizing current, operates in response to the control voltage from the monitor circuit to interrupt the anodizing current when the resistance of the resistor reaches the desired value.

BRIEF DESCRIPTION OF THE DRAWING The nature of the present invention and its various advantages will appear more fully by referring to the following detailed description in conjunction with the appended drawing, in which:

FIG. l is a schematic drawing of a feedback anodizing circuit constructed in accordance with the invention; and

FIG. 2 is a schematic drawing of an alternate embodiment of the resistance sensing circuit constructed in accordance with the invention.

DETAILED DESCRIPTION Referring to FIG. l, a thin-film resistor 10 is connected into one arm of a bridge circuit 11. The resistor 10 may comprise a resistive pattern of tantalum which has been sputtered onto an insulative substrate. The resistance of the resistor 10 is to be increased to a preselected value by anodizing the tantalum film to reduce the cross section of conductive material and thereby increase its resistivity. The output of the bridge circuit 11 is connected to a monitor circuit 12 lwhich controls the rate at which anodizing current is delivered by a current supply circuit 13.

The bridge circuit 11 includes a first resistive arm 14 and a second resistive arm 15 connected in series between ground potential and a negative 6 volt power supply 9. The bridge is completed by a third resistive arm 16 connected in series with the thin-film resistor 10. A first operational amplifier 19 is connected between points 28 and 18 of the bridge and acts as a null detector to indicate the degree of deviation of the bridge from a balanced condition. The amplifier 19 has a very high input impedance to avoid loading the bridge circuit and lprovides essentially unity gain. The positive output signal of the detector amplifier 19 is connected to a first input resistor 20 of a second non-inverting operational amplifier 22. The negative input terminal of the amplifier 22 is grounded through a second input resistor 21. The amplifier 22 has a feedback resistor 23 connected from its output to its input to provide a large Voltage gain due to the large ratio of the feedback resistor, which is in the range of l megohm, to the input resistor, which may be aproximately 3K ohms.

The second amplifier 22 couples the bridge circuit 11 to the monitor circuit 12 in that its output is connected by an input resistor 24 to the base of a first transistor 25. A diode 26 is connected between the base and emitter electrodes of the transistor 25 to protect the transistor base-emitter junction from excessive negative voltages. The emitter of the transistor 25 is grounded. A biasing battery 27 is included to hold the base voltage of the transistor 25 approximately 0.7 v. positive with respect to the emitter to overcome the base emitter voltage drop which is characteristic of conventional silicon transistors. The battery 27 will allow the transistor to begin and cease conduction when the bridge is balanced and the amplifier 22 produces a zero output voltage. The collector of the first transistor 25 is connected to a second transistor 28 through a current limiting resistor 29. A biasing resistor 32 is provided between the base and emitter electrode of the second transistor 28. The emitter of the second transistor 28 is biased by a positive 20 volt source 30.

The output of the second transistor 28 is connected to drive a first lamp 34 through a load resistor 35. A second monitor lamp 3-6 is also connected across the output of the second transistor 28 to provide an indication to the operator of whether the circuit is anodizing. For the operating voltages herein described, the lamps are preferably of the type rated at 26 volts. The first lamp 34 is in optical communication with a photo-resistor 37; both of which are sealed in a light-tight enclosure 38. As the illumination intensity of the lamp 34 increases the resistance of the photo-resistor 37 decreases. Although a number of different photo-resistors could :be used, one similar to the type LDR 25 photocell manufactured by the Delco Corporation is preferred. This type of photoresistor has a resistance which varies from greater than one megohm 'when dark to approximately 500 ohms when illuminated by the 26 volt lamp 34 operatin-g at 2O volts. Further, the photo-resistor is rated at maximum operating values of 200 volts, l ampere and 25 watts power dissipation.

The photo-resistor 37 is connected in series with a negative G-volt power supply 40 through an anodizing switch 39. The photo-resistor 37 is also connected to a summing point 42 through a first |voltage dividing resistor 43. The summing point 42 is in turn connected to the positive -volt power supply 30 through a second voltage dividing resistor `44. The summing point 42 serves to deliver anodizing current to a cathode electrode 45 through a current control potentiometer 46 and a diode 47. The cathode electrode 45 is in electrical anodizing contact with the thin-film resistor 10 through a body of electrolyte 48 which may be composed of a viscous solution of 0.01% citric acid. An incandescent lamp 49 having a tungsten filament and a load resistor 50 are series connected from between the resistor 43 and the photo-resistor 37 to ground. The lamp and resistor provide a suitable high resistance load for the power supply under normal anodizing conditions and, because of the characteristics of the tungsten, also serve to shunt the small value of anodizing current near the end of the anodization process.

In operation, the variable resistor -16 of the bridge circuit 11 is set equal to the final desired resistance of the thin-film resistor -10 and the anodizing current switch 39 is closed. When the thin-film resistor 10 is anodized to equal the variable resistor 16 the bridge is balanced and the potential at points 17 andv 118 will be equal. Initially, the bridge is unbalanced, because the resistance of the thin-film resistor 10 is too low, and there is a potential difference between points 17 and l18 which is proportional to the degree of deviation of the thin-film resistor 10 from the standard resistor 16. The detector amplifier -19 connected across points 17 and 18 produces an output voltage which is indicative of the degree of imbalance in the bridge. The power amplifier 22 delivers current to dri-ve the transistors 25 and 28 which deliver an output voltage to illuminate the lamps 34 and 36. The intensity of illumination of the lamps 34 and 36 vary in direct proportion to the degree of imbalance in the bridge circuit 11.

The variation in illuminating intensity of the lamp 34 will cause the resistance of the photo-resistor 37 to vary in the same proportion. The variation in resistance of the photo-resistor 37 varies the voltage drop across the resistor and therefore, the potential at the summing point 42. Thus, when the lamp 34 is at maximum intensity, the resistance of the photo-resistor 37 is small and a large negative voltage due to the negative 150 volt source is applied to point 42. A negative voltage at point 42 produces an anodizing current which flows through the cathode electrode 45 and anodizes the thin-film resistor l10 to increase its Value. As the bridge moves toward balance, the lamp 34 becomes darker, the resistance of the photo-resistor 37 increases, the Voltage at point 42 becomes less negative, and a smaller amount of anodizing current is delivered to the cathode electrode 45 so that the rate of anodization is decreased.

When the resistor 10 reaches the desired value the bridge is balanced and the lamp 34 is dark because the transistors 2S and 28 have ceased to conduct. At this point, the resistance of the photo-resistor 37 is very large and the potential at the summing point 42 has become slightly positive due to the voltage divider resistors 44 and 43 and the positive 20-volt source 30. This positive voltage back-biases the diode 47 and insures that no more anodizing current can flow to the cathode electrode 45.

A characteristic of the bridge and monitor circuits 11 and 12 is that as the bridge approaches balance, the anodizing current becomes very small. This means that the last small increments of change in value of the resistor 10 takes place very slowly. For this reason, it is desirable to stop the anodizing current altogether when it is so small that its effect on the resistor 10 is negligible. For this purpose the incandescent lamp 49 is connected from ground through a variable resistor 50 to a point between the photo-resistor 37 and the summing point 42, to act as a voltage-dependent resistance device. That is, as the voltage across the lamp becomes very low the current which the lamp draws increases. Since the tungsten filament of Athe lamp 49 is hot, when the voltage at the summing point 42 is large, the resistance of the filament is high. As the voltage at the summing point 42 becomes comparatively low, the filament begins to cool and its resistance decreases rapidly. The lower resistance tends to shunt the remaining amount of anodizing current to ground. This effect serves to stop the flow of anodizing current as the value becomes negligible.

Another significant feature of the circuit resides in the manner in which anodizing current is applied to the thinfilm resistor 10 without appreciably affecting current flow in the bridge circuit. When the bridge is unbalanced as the anodization process begins, point 17 is typically at a potential of minus 3 Volts While point 18 is at a potential of approximately minus 2 Volts. As the negative cathode 45 applies anodizing current to the resistor 10, the current flow therethrough creates a voltage drop and point 18 tends to become slightly more negative. That is, the flow of anodizing current tends to create a negative feedback effect and dri-ves the bridge towards a balanced condition. It is this effect which allows a resistor in a bridge circuit to be measured and anodized simultaneously.

The gain of the amplifier 22 and the driving current resistor 24 may be adjusted so the amplifier and the transistors 25 and 28 operate in a condition of saturation until the bridge is very nearly balanced. This condition results in maximum anodizing current flow and enables the total anodization time to be reduced. As balance is closely approached, the components come out of saturation and reduce the anodizing current. Since part of the appearance of bridge balance is due to a Voltage drop in the resistor 10 resulting from anodizing current, the bridge will move slightly away from `balance as the anodizing current is reduced. The small remaining portion of anodization will take place in the conventional manner described above. The relatively long response time of the photo-resistor 37, that is, between an illumination change and a corresponding change in resistance, prevents the entire circuit from going into oscillation during the slightly unstable condition as the bridge nears balance.

An alternate measuring circuit 11 which may be substituted for the bridge circuit 11 of FIG. 1 is shown in FIG. 2. A negative 6-volt power supply 9 is connected across the variable standard Value of resistance 16 and the thin-film resistor 10 to be anodized. The negative input of the operational amplifier 19 is connected to the negative 6-volt source through a 25K ohm input resistor 14. A 25K ohm feedback resistor 15 is connected from the output to the negative input of the amplifier 19 to give unity gain. The positive input terminal of the amplifier 19 is connected to point 1S between the standard resistor 16 and the thin-film resistor 10. The output of the amplifier 19 is to be connected to the input resistor 20 (FIG. l) of the second amplifier 22.

A characteristic of the operation amplifier 19 is that it produces an output voltage which, through its feedback resistor 15, tends to make the voltage on the positive and negative input terminals equal. For example, when the value of the thin-lm resistor 10 is less than the standard resistor 16, the potential at point 18 is less than one-half of -6 volts. Assuming point 18 is at -2 volts, the output voltage of the amplifier 19 must go to +2 volts in order to make both the positive and negative input terminals both equal to -2 volts. The positive output voltage drives the monitor circuit 12 (FIG. 1) to control the anodization current. Similarly, when the thiniilm resistor is equal to the standard resistor 16 the potential at point 18 will be -3 volts. Since the positive input terminal of the amplifier 19 is -3 volts, the output potential must go to zero volts so that the negative input terminal will also be -3 volts. Thus, the amplifier output voltage of the amplifier is zero when anodization is to cease.

It is to be understood that the above described embodiments are simply illustrative of the invention and that many other embodiments can be devised without departing from the scope and spirit of the invention.

What is claimed is:

l1. A circuit for controlling the rate at which a thinlm resistor is anodized to increase the resistance of said resistor to a preselected value, comprising:

a Wheatstone bridge having said thn-lm resistor connected as a iirst arm, a resistor equal to said preselected value connected as a second arm, and means for producing an output signal which is proportional to the difference between said rst and second arms;

a control lamp connected to the detector of said bridge having an illumination intensity which is proportional to the outputsignal of the detector;

a source of anodizing voltage;

means, including a cathode electrode, for applying anodizing current from said source to said thin-film resistor to anodize said resistor and increase its resistance; and

a photo-sensitive resistor connected between said voltage source and said means for applying anodizing current, said photo-sensi-tive resistor being optically connected to said control lamp to increase its resistance and limit the anodizing current in response to a decrease in illumination intensity of said control lamp.

2. A circuit as set forth in claim 1 which also includes:

voltage-dependent resistance means connected from between said photo-sensitive resistor and said means for applying anodizing current, to ground, said voltage-dependent resistance means having a high resistance when the voltage thereacross is large, said resistance decreasing to shunt the flow of anodizing current as the voltage thereacross decreases.

3. A circuit as set forth in claim 1 which also includes:

an incandescent lamp connected from between said photo-sensitive resistor and said means for applying anodizing current, to ground, said lamp including a filament having a high resistance when heated which filament cools and decreases in resistance to shunt the vllow of 'anodizing current as the resistance of the photo-sensitive resistor becomes very large.

4. A circuit for controlling the rate at which a thin-film resistor is anodized to increase the resistance of said resistor to a preselected value, comprising:

means for producing a control signal which is indicay.tive of the diierence between Ithe resistance of said thin-film resistor and said preselected value;

means responsive to said control signal producing means ,for producing radiation having an intensity which is dependent upon the value of said control signal;

a source of anodizing current;

means for applying anodizing current from said source to said 4thin-film resistor to anodize said resistor and increase its resistance; and

means responsive to the radiation produced by said radiation producing means for varying the anodizing current delivered to said current applying means by said current source.

f5. A circuit for controlling the anodization of a thinlm resistor, as set forth in claim 4, in which:

said radiation responsive means includes radiation sensitive resistance means connected between said current source and said means for applying anodizing current, said radiation sensitive resistance means being coupled to said radiation producing means to increase the value of said radiation sensitive resistance and limit the anodizing current in response to a corresponding change in radiation intensity.

6. A circuit for controlling the anodization of a thinlm resistor, as set for-th in claim 5, in which:

said radiation producing means is a control lamp; and

said radiation sensitive resistance means is sensitive to visible light.

References Cited UNITED STATES PATENTS 3,282,821 11/1966 Cistola 2014-228 3,341,444 9/1967 La Chapelle 204-228 `3,341,445 9/1-967 Gerhard 204-228 3,481,843 12/1969 Koo 2o422sx FOREIGN PATENTS 557,471 5/1958 Canada y 204-228 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

